All UK MVNOs piggyback off either Three, Vodafone, O2 or EE.
Of the ones recommended by Which?, Talkmobile use Vodafone's network, Giffgaff, O2's, Smarty use Three's (in fact Smarty are owned by Hutchison who operate Three), 1p Mobile use EE and Lebara use the Voda network.
The other one...